A screw compressor characteristically includes a rotor housing which defines a working chamber in which cooperating screw rotors are disposed. The working chamber is closely dimensioned to the outside diameter and length dimensions of the meshed pair of screw rotors disposed therein and therefore is a volume generally shaped as two intersecting parallel cylindrical bores. The male rotor characteristically consists of adjoining lobes having leading and trailing flanks which are convex. The female rotor consists of lobes having leading and trailing flanks which are concave. Conventionally, it is said that the major portion of the male rotor lobes and grooves are located outside of the pitch circle of the male rotor. Likewise, it is conventionally stated that the major portion of each female rotor lobe and groove is found within the pitch circle of the female rotor. The wrap angle of the male rotor is typically less than 360.degree..
A suction port is disposed at one end of a screw compressor and is the inlet through which a gas to be compressed is received between the screw rotors within the working chamber of the compressor. As the rotors rotate and mesh, the volume in which such gas is initially received between the rotors decreases and is moved axially between the rotors toward the opposite end of the working chamber where a discharge port is located.
A primary concern in the design of screw compressors, as with other compressors, is the maximization of compressor efficiency. Screw compressor efficiency relates directly to the design of the helical screw rotors disposed within the working chamber of the compressor. The efficiency of a screw rotor set depends upon both the rotor profile chosen and the degree of precision to which the rotors can be manufactured. The rotor profile should preferably minimize the leakage paths between the rotors and the rotor housing while not being so complicated as to be entirely uneconomical to design and manufacture. A screw rotor profile is the profile of a rotor set taken in a plane transverse to the parallel axes of male and female rotors.
As is recognized throughout U.S. Pat. No. 4,412,796, the screw rotor profile art is a crowded and well-developed one. Yet, that patent is correct in its assertion that while improvements in the art come in small increments, such refinements, whether they be in the area of improved compressor performance, energy savings, the methods by which profiles are obtained or in the area of improved ease of rotor manufacture, represent meritorious and innovative advances in the screw compressor art.
There is no lack of teaching concerning the historical development of screw rotor profiles. U.S. Pat. Nos. 3,423,017; 4,460,322; and 4,508,496 all provide valuable background and insight into screw rotor profile design. Such names as Lysholm, Whitfield, Nilsson, Persson and Schibbye were preeminent in the development of basic screw rotor technology in the 1940's, 1950's and in the 1960's. Seemingly, the primary focus in the 1970's and 1980's has been on the refinement of screw rotor profiles so as to obtain enhanced machine efficiencies and/or the development of profiles which are more easily and economically manufactured in quantity yet which are acceptably efficient.
One rotor profile feature which has been recognized of late as being advantageous to screw rotor profile design is the use of involute curves to define at least a portion of the rotor profile. The use of such curves is advantageous both from the standpoint of ease of rotor manufacture and compressor efficiency. Perhaps the first to identify the advantages of involute curved surfaces with respect to screw rotor profiles was Menssen whose U.S. Pat. No. 4,028,026 has its roots in a German patent application filed in 1972. Menssen's U.S. patent discloses a screw rotor profile having involute curved portions on both the leading and trailing flanks of both the male and female rotor. It does, however, also teach a profile replete with corners and discontinuities between lobe portions. Menssen makes a brief reference to the generation of his profile by a set of racks which are coincident along 75% of their length. How or why such racks are developed is not discussed in any detail. Ingalls' U.S. Pat. Nos. 4,053,263 and 4,109,362 have their roots in a patent application filed in 1973. These relatively early U.S. patents to Ingalls also relate rotor profiles containing involute portions. Three significantly more recent U.S. Pat. Nos. 4,445,831, again to Ingalls, and 4,412,796 and 4,508,496 both to Bowman, teach the use of involute curved portions on a screw rotor profile. None of the aforementioned patents, however, teaches the advantageous rotor profile disclosed herein which is generated in an uncomplicated fashion and the use of which results in efficient rotor sets capable of economical manufacture in quantity.